Eligible cards can display an illustrative image for the concept the word represents. This article explains which cards are eligible, how to request an image, what happens during generation, and where images appear across the app.
Note
Vocabulary images are a flangu Pro feature.
Which cards are eligible
Not every card can have an image. Eligibility is determined by part of speech.
Cards classified as nouns, proper nouns, verbs, or adjectives are eligible. All other parts of speech (adverbs, prepositions, conjunctions, sentences, questions, and the rest) are not.
The image icon is only shown on eligible cards. If you do not see an image icon on the front panel of a card, the card's part of speech is the reason.
An image also requires the card to be linked to flangu's shared translation cache. Cards created in 
Translation mode are linked. Cards created in Manual mode, and cards whose content you have edited, are usually not linked, and show "Image generation not available for this word." See What happens when you save.
How to request an image
On an eligible card, the front panel shows an 
image icon in the icon row. Tap it to request an image.
Generation runs in the background. The icon pulses while the request is in progress. A new image usually takes 15 to 20 seconds. When many images are being created at the same time, it can take longer. An image that already exists appears almost instantly (see Shared images).
When generation succeeds, the icon changes to a green checkmark and the image appears in the image section of the card.
Image generation requires an internet connection. Offline, the image section shows "Image generation requires an internet connection." Images that are already on a card stay visible offline.
Leaving the card during generation
You do not have to wait on the card. If you open another screen while an image is being generated, generation continues. When you return to the card, the icon pulses again and the image appears once it is ready.
If you pick an alternative translation while an image is being generated, that request is dropped, because the image was for the previous word. Tap the image icon again to get an image for the new translation.
Shared images
Images are generated once per concept and shared across all users. If another user has already generated an image for the same concept, the image appears without a wait, whatever language pair their card uses. "pies" in a Polish deck, "Hund" in a German deck, and "dog" in an English deck all share one image.
An image matches the meaning of the card, not just its spelling. flangu uses the translation and the card's definitions to work out which meaning is meant. A card for "seal" the animal and a card for "seal" the stamp get different images, and so do "crane" the bird and "crane" the machine.
This works the same way as the translation cache for linguistic metadata. A common word will almost always load instantly. A rare word may take longer while its image is created for the first time.
If an image turns out to be wrong for its word, flangu can remove it. It then disappears from every card that uses it the next time those cards sync, and you can request a new image for the card.
Tapping the icon again
Once an image is displayed, tapping the image icon again hides it. The icon returns to its default state but becomes non-interactive while the image is active. To show the image again, tap the icon once more.
Where images appear
The image section behaves differently depending on which screen you are viewing.
Translator screen
On the Translator screen, the image displays inside the front card panel: above the icon row and below the main word text, in the same area as definitions, examples, and other metadata sections.
Card detail and review screens
On the card detail screen and during a review session, the image is displayed at the top of the front card panel, above the word. When an image is active, the word text moves below it.
When an image is not available
Generation can fail for three reasons.
No clear concept. Before generating an image, flangu matches the word to a known concept in the meaning the card uses. If the word is too abstract to draw, or no matching concept exists, the message reads "No clear visual concept could be identified for this word." Requesting the image again gives the same result for 30 days. After that, you can try again.
Content restrictions. Some words cannot be illustrated under the image service's content rules. The message explains this. Requesting the image again gives the same result.
The service is temporarily unavailable. If the image service is down or busy, you will see a message asking you to try again later. This is temporary.
In every case, the card itself is not affected. All other metadata, audio, and functionality remain available.